1. Field of the Invention
The present invention relates to a method of performing land seismic data acquisition, and a seismic cable and a cable spool vehicle therefor. More particularly, the invention relates to a method and apparatus for mechanically deploying seismic cable with attached sensors according to a desired geophysical spread and for allowing subsequent mechanically pick-up of the seismic cable together with the sensors.
2. Description of the Prior Art
In most conventional land seismic data acquisition, individual analog seismic sensors, so-called geophones, typically having one or more spikes attached to their cases, are planted in groups in the ground with the center of gravity of the group along a seismic survey line. In order to ensure a proper and stable acoustical coupling of the geophones with the ground, each geophone is normally driven into the soil by a vigorous blow on its top applied by a seismic crew member. Before the planting of each geophone, the crew member has to estimate the desired proper position (with respect to geophysical requirements) for the geophone, which is usually realized by simple visual estimation of the geophone position versus a survey peg placed in the center of gravity of the geophone group. Additionally, the crew member has to plant the geophone so that it is vertical. Grouping the analog output signals of a certain number of geophones and adding their output signals permits the noise signals that are normally superimposed on the seismic signal, such as the horizontally traveling wave (ground roll) and various types of random, incoherent noise (e.g. wind, rain, scratching of the geophone caused by moving plants, oscillation of the geophone cable) to be significantly reduced. Each such group of geophones is connected to seismic data acquisition and recording units.
Conventional land seismic data acquisition demands a large number of geophones and cables, together with a large crew and considerable logistics, to lay out the desired geophysical spread and to pick it up again after the seismic survey. Such operations are very time consuming and very expensive.
In order to reduce the setting up time expense of land seismic data acquisition, in particular the number of crew members required, it has been proposed in the past to apply a marine seismic survey technology, (i.e., streamer technology), to land seismic operations. Here, a multiplicity of sensors are arranged in a line and, instead of being "statically" laid out by being individually planted, are dragged over the ground along a desired seismic survey line.
One such known land seismic streamer comprises a flat band-like device with incorporated interconnected single-component analog geophones, which, like a marine streamer, was towed by a vehicle, and dragged over ground covered with snow and ice. On the one hand, the flatness of this streamer helped to avoid turning over the streamer and to keep the geophones in a proper upright position. On the other hand, the required good acoustical coupling of the streamer to the ground surface could not be reliably achieved. The flat streamer, because of being dragged and stretched, could not properly follow the contours of the ground, and tended to be drawn over the high points of the ground and to remain stretched without touching the lowerpoints. Furthermore, the flat streamer was very sensitive to wind, which caused significant noise that became superimposed on the desired seismic signal. Another drawback of the flat streamer was that its sensors were incorporated into more or less flexible band sections that alternated with stress compensating members. Replacement of a defective geophone required the replacement of a complete streamer section, a costly action. It is also evident that when such a streamer is pulled and dragged over a dry ground surface, (ie sharp-edged stones, sand grains etc), it will be heavily affected by abrasion effects and, in the case of sand, by electrostatic effects.
Another prior art land streamer device comprises a main cable with integrated stress compensation members that is also towed behind a vehicle and dragged over the ground. Unlike the aforementioned flat cable streamer, this other streamer did not contain geophones that were mounted as part of the main cable, but rather geophones that were attached to additional secondary cables connected to the main cable via outlet connectors at regular intervals. In order to compensate for improper sensor orientation, the geophones used in this streamer were single component gimbal-mounted analog sensors. But such geophones only allow compensation for a single axis inclination. And the drawbacks of such a towed land streamer are evident. This streamer too does not always follow the surface contours, or lay properly on the ground or provide sufficient acoustical coupling. Dragging such a streamer over pebbly or rocky ground risks getting it stuck at any restriction in its way. A stuck but still pulled streamer may easily be damaged, destroyed or become a danger for personnel in the field if it unexpectedly frees itself because of the pulling force. The useful life of such a steamer is also very short in view of the abrasion caused by dragging it over the ground.